The invention relates to a rotor blade design for a Wells turbine, in particular for use in a wave powerplant
Wells turbines are known—reference is made to GB 1574379 A in this regard, for example. Turbines of this type are designed for a bidirectional, essentially axial inflow, the rotor of a Wells turbine maintaining the revolution direction independently of the direction of the inflow. The rotor typically comprises a hub having rotor blades which are distributed around the outer circumference and point radially outward. The rotor blades are designed symmetrically to the rotation plane, in general drop-shaped airfoil profiles being used as the profiles. Furthermore, the rotor blades are typically linked on rigidly. However, designs having adjustable-angle rotor blades are also known, the above-mentioned symmetry plane of the rotor blades lying in the rotation plane in the neutral position in this case.
Furthermore, U.S. Pat. No. 5,191,225 A discloses a wave powerplant, in which a two-stage Wells turbine is received in a flow duct and is driven by a bidirectional airflow. In a wave powerplant of this construction, energy is obtained from an oscillating water column. For this purpose, a wave chamber is provided, which has an inflow opening lying below the water level. If a wave breaks against the outer wall of the wave chamber, an flow of sea water occurs into the interior of the chamber, whereby the water level in the chamber rises. The water level will drop accordingly in the event of an outgoing wave, so that an oscillation movement of the water column in the wave chamber results, which approximately corresponds to the wave frequency. An air mass is enclosed above the water level in the wave chamber, which is connected to the environmental atmosphere by a limited ventilation duct. In accordance with the oscillation movement of the water body in the wave chamber, the air mass located above it will be subject to a pressure variation, so that a continuously changing, bidirectional airflow of high velocity arises for pressure compensation in the ventilation duct, which can be used to acquire electrical energy.
A possible profile course for the rotor blades of a Wells turbine is given by the four-digit NACA series. Thus, the NACA 0012 profile describes a profile contour having a thickness of 12% in relation to the chord length. The profile chord and thus the line of symmetry of the profile run parallel to the rotation plane of the turbine. During operation of the Wells turbine, an effective inflow angle against the profile of the respective turbine blades exists, which results through the vector addition of the peripheral velocity and the inflow velocity of the driving airflow. The inflow angles are typically small, so that the symmetrical profile acts in terms of an airfoil and the forces engaging on the aerodynamic center have a partial component in the tangential direction in relation to the hub of the Wells turbine, which is used for propulsion. The further aerodynamic forces perpendicular to this tangential direction are to be absorbed via the turbine bearings.
The difficulty results for Wells turbines, in addition to the problems of high noise emission and poor startup behavior, that with increasing inflow angle, typically from 13°, a flow stall can occur at the turbine blades. Such a large inflow angle can occur for large flow coefficients, if a rapid increase of the inflow velocity exists because of a strongly varying airflow and the rise of the peripheral velocity of the Wells turbine does not occur rapidly enough, and/or it already runs at its rated speed.